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Easy "All of Us" achievement or trophy

In the final level, after opening the wall, you will have to follow hallways until reaching the Cyberdemon. Remain on your left at all times during this time to find the secret room. The room is located just before meeting the Boss. That room contains the last PDA, with words from the iD developers.

Easy "Big Boy" achievement or trophy

When your Soul Cube is activated, use it on the Cyberdemon. Imps and Maggots will also spawn from the center of the room, allowing the weapon to be refilled. Avoid the rockets from the Boss. Consider running around the center.

Easy "Bot Buddy" achievement or trophy

To keep a sentry bot alive, play under the Recruit difficulty setting. Kill enemies before they attack your sentry bot. Sentry bots can be found during the following levels.

Doom 3 "EnPro Plant, Communications", where you can take a new one if your buddy is destroyed.

RoE "Erebus - Level 6, Phobos Labs - Sector 3"

The Lost Mission "Underground - Sector 2"

Easy "Cookie Stealer" achievement or trophy

The Guardian is located at the end of the "Hell" level. Start by taking down the light seekers that are flying around. The Guardian is blind and uses them to locate you. Afterwards, his back will open and an orb will leave it. Shoot it with everything you have. Do this at least three times to defeat The Guardian.

Easy "Gimme Power!" achievement or trophy

The Berserk Hunter is found at the end of "Erebus - Level 6". He can only be damaged when his chest opens to expose his heart. Use the artifact to slow time and shoot it. You can reload your hellish weapon with the bodies in the area, if the Hunter did not make them disappear. When you cannot do anything else, run and wait until an opening occurs. This must be done at least three times to defeat him.

Easy "Gimme Time!" achievement or trophy

The Helltime Hunter is found in "Erebus - Level 2". To defeat him, use the IPL to grab the orbs fired by the Martian turrets then send them on the Hunter while he is in front of the portal. He will also moving around until he is close enough to attack you. Make sure to escape those attempts. You will also have to avoid the plasma balls he throws at you. Do not shoot him with your weapons, as they will have no effect.

Easy "Ready for Action!" achievement or trophy

Save the scientist stuck in the reactor room in "Alpha Labs - Level 4". He will open a room full of items and Abrams's PDA (the Security Chief in Delta). You will find the code for his office on the PDA. Once in Delta 2a, move the crates to reach a ledge to arrive in a hallway with the famous office. Enter the code then go to the wall and activate the panel to open the display case with the BFG-9000. Note: Abrams says that his code is "901" but security answers that include "0" are prohibited, so they replaced it with a "3".

Easy "Shocking!" achievement or trophy

The Invulnerability Hunter can be found in Phobos Labs - Sector 2. To defeat him, grab the orbs he throws at you with the Grabber. Send them back at him to hurt him. If you are upstairs, you cannot be injured by his shockwave attack, however, you will have less room to move. He will eventually recharge himself with all the lighting arcs. During this time, open the remaining tubes. Run for the one with a green panel, then open it by activating it. Use powerful weapons to destroy its interior. Do this three times on the number of remaining tubes. Afterwards, the Hunter will be defeated and transfer the Invulnerability into the artifact.

Easy "You Laugh, It Works" achievement or trophy

Before entering the main portal chamber, notice the skeleton in the corner. Approach it and use your flashlight to see the message "You Laugh, It works" written in blood next to him.

Achievements

Complete the following achievements to unlock Xbox Live Gamerscore points.

DOOMed Recruit (5 points): Complete the DOOM 3 single player campaign on Recruit.

DOOMed Marine (10 points): Complete the DOOM 3 single player campaign on Marine.

DOOMed Veteran (15 points): Complete the DOOM 3 single player campaign on Veteran.

DOOMed Nightmare (20 points): Complete the DOOM 3 single player campaign on Nightmare.

DOOMed Collector (15 points): Collect every PDA in DOOM 3.

I Like to Watch (15 points): Find all video logs in DOOM 3.

That was Close! (15 points): Kill an enemy with 1 health remaining in DOOM 3, RoE, or Lost Mission.